In a paper making machine, water is removed from a wet web of paper fibers between a series of mutually opposing rolls by compressing the paper fiber web, and press felt in the form of an endless belt having a relatively large width is wrapped around such rollers. The press felt is fabricated by passing woven ground fabric in the form of an endless belt around rolls including a motor driven roll and guide rolls, and entangling fibers of fiber web consisting of layers of woollen or synthetic fiber web with the ground fabric by needling as the ground fabric is passed under a needle head along with the fiber web placed thereon.
During the process of needling, for the fibers to be favorably entangled with the ground fabric, it is necessary to adjust the stroke of needling, the density of needling and the feed speed of the ground fabric according to the number of turns which the ground fabric has made around the feed and guide rolls. Since the press felt must be highly smooth along the circumferential direction thereof without any steps or local irregularities, it is difficult to determine how many turns the press felt has made at any particular given time. Furthermore, since the time point of adjustment must coincide with the point of transition from one layer of fiber web to another, the detection of the rotation of the drive roll would not allow a sufficiently accurate detection of the position of the press felt during the process of needling.
Conventionally, the operator placed a red thread or made a red mark with dye or ink in the ground fabric to identify a reference point on the press felt, and manually made necessary adjustments by visually determining the number of turns the ground fabric had made. Therefore, the operator was required to count the number of turns the press web had made and quickly make necessary adjustments upon detection of such a marker. This required a high level of concentration, and the quality of the press felt was highly dependent on the quality of the operator. Therefore, there has been a strong demand to automate the process of felt fabrication by needling.
For automating the process of needling, it is preferable to be able to use a marker which can be easily detected with a sensor. However, conventionally known markers were inadequate because they tended to be quickly damaged by the process of needling in which the entire press felt including the sensor is repeatedly pierced by needles, and were rendered useless in a very short time.
In Japanese patent laid open publication No. 03-124866 filed jointly by the applicant of this application and two other applicants, it is proposed to prepare a magnetic marker by arranging a plurality of magnetized fibers parallel to each other on a two-sided adhesive tape, and securing it to a lateral fringe of the ground fabric of press felt so that the movement of the ground fabric may be detected with a magnetic sensor during the process of needling, and the process of needling can be automated by using an output signal from the magnetic sensor which accurately indicates the movement of the ground fabric. Further, since the marker basically consists of magnetized fibers, the needling process is not hampered by the presence of the magnetic marker, and the marker can withstand the repeated piercing by the needles.
However, since the magnetized fibers tended to be integrally combined with the ground fabric along with the fibers of fiber web as a result of the process of needling, the marker was not suitable for repeated use. Since the magnetized fibers typically consisting of amorphous alloy are highly expensive, it is more desirable if the magnetic marker can be used repeatedly.